[Bug tree-optimization/81436] New: missing -Wstringop-overflow on strncat to a zero-size buffer

["msebor at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81436

            Bug ID: 81436
           Summary: missing -Wstringop-overflow on strncat to a zero-size
                    buffer
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

Strncat always appends a terminating NUL to the end of the copied string. 
Therefore, it's never safe to call it with a destination of zero size.  Such
calls should be diagnosed by -Wstringop-overflow but in GCC 7.1 they are not.

$ cat a.c && gcc -O2 -S -Wall -Wextra -fdump-tree-optimized=/dev/stdout a.c
void sink (void*);

void f (const char *s, int n)
{
  char d[1];

  __builtin_strncat (d + 1, s, n);   // missing -Wstringop-overflow

  sink (d);
}

;; Function f (f, funcdef_no=0, decl_uid=1818, cgraph_uid=0, symbol_order=0)

f (const char * s, int n)
{
  char d[1];
  long unsigned int _1;

  <bb 2> [100.00%] [count: INV]:
  _1 = (long unsigned int) n_2(D);
  __builtin_strncat (&MEM[(void *)&d + 1B], s_4(D), _1);
  sink (&d);
  d ={v} {CLOBBER};
  return;

}